In reprography, halftone color separations are used, as is known, as copy originals for making offset or letterpress printing plates. Before the printing plates are exposed, the color separations are proofed for this purpose with the aid of color proofing processes to determine whether the ultimate printing result will be a correct tonal value reproduction of the original.
Widely used, standard specifications, for example, for a high quality offset print, require a resolution of 2%-98% dots in modern printing machines, and 1%-99% dots at a screen count of 60 lines/cm. At this line count, it is especially difficult to achieve satisfactory resolution of the pointy 2% dots in the highlights and the 98% dots in the shadows. Moreover, for good tonal reproduction, it is of considerable significance for halftone dots of the same size to be sharply defined over the entire surface area and to be reproduced uniformly, that is, without variations in size.
Such color proofing processes use light-sensitive recording materials that produce an image by the difference in tackiness between the exposed and unexposed areas of the light-sensitive layer. Thus, for example, German Patents 12 10 321 (Burg, U.S. Pat. No. 3,060,024), 19 04 058 (Chu, U.S. Pat. No. 3,620,726), 19 04 059 (Boyd, U.S. Pat. No. 3,582,327), and 20 04 214 (Chu, U.S. Pat. No. 3,649,268), Buzzell, U.S. Pat. No. 4,356,253, and EP-A 02 43 933 (Bauer, U.S. Pat. No. 4,892,802) disclose a reproduction process in which a tacky photopolymerizable recording material comprising a support and a photopolymerizable layer containing at least one addition-polymerizable monomer and a photopolymerization initiator is hardened by imagewise exposure so that the exposed image areas lose their tackiness. The latent image is then rendered visible by the application of a suitable toner that adheres only to the unexposed, tacky areas. Toner remaining on the exposed, non-tacky image areas can be removed after application. This process yields positive, optionally colored, images of the original, and these images resemble in appearance, images produced by color printing.
Similarly, negative images of the original are obtained by using the light-sensitive materials described, for example, in German Patents 27 58 209 (Abele, U.S. Pat. No. 4,243,741), 30 23 247 (Abele, U.S. Pat. No. 4,346,162), and 34 29 615 (Grossa, U.S. Pat. No. 4,604,340). The light-sensitive component of these recording materials is either a dihydropyridine compound or a system of a dihydropyridine compound and a hexaaryl bis-imidazole compound.
Toners comprising predominantly finely divided powders can be applied by dusting the toner on the imagewise exposed surface. Alternatively, the toner can be loosely bonded to a special support and transferred by bringing the support into contact with the imagewise exposed layer.
Such transfer layers are substantially more advantageous, because they are (a) significantly simple to handle, (b) cleaner, and (c) environmentally safer. Pigmented transfer layers are described, for example, in German Patents 12 05 117 (Burg, U.S. Pat. No. 3,060,025) and 29 49 462 (Neiss, U.S. Pat. No. 4,275,140). These transfer layers are undesirable because they do not satisfy the stringent requirements of color proofing processes. DE-C. 36 25 014 (Frohlich, U.S. Pat. No. 4,806,451) , DE-C 37 06 528 (Delaney, U.S. Pat. No. 4,902,363), DE-C 39 41 446 (U.S. Pat. No. 5,126,226), DE-C 39 41 493, and EP-A 03 65 361 (Platzer, U.S. Pat. No. 4,935,331) describe color proofing processes using transfer layers specifically developed to meet the needs of the printing industry.
Although these known processes can indeed fulfill the most important requirements of the printing industry, e.g., high resolution, good tonal value reproduction, and low dot growth, the problem of background fog or stain has not yet been solved. This defect depends on the greater or lesser transfer of the color pigments of the transfer layer onto the non-tacky areas of the recording materials, i.e., areas that actually should not be toned. Background stain occurs particularly when the light-sensitive recording material does not show good differentiation in the tackiness of exposed and unexposed areas. Background fog or stain is also very pronounced in many of the colored transfer layers, particularly if the transfer layer components are not optimally coordinated with each other. Background stain can in fact be prevented by modifying the process requirements in manufacturing and/or processing, that is, in laminating the transfer layer onto the recording material. However, this demands special, technically expensive and time-consuming measures for each individual material.
There is a need for a proofing process that (a) can attain the high resolution of 1%-99% dots required by the printing industry, (b) is outstanding for high uniformity of halftone values overall and for very low dot growth, (c) can be used for different tonable, light-sensitive layers, and (d) produces color proof prints without background stain.
Accordingly, these needs are met using the process of the invention.